The present invention relates to an ignition system of AC continuous discharge type used for an internal combustion engine.
In the prior art, an ignition system of AC continuous discharge type such as disclosed in JP-B-No. 62-6112 (U.S. Pat. No. 4,356,807) has been suggested in which an electric current is supplied alternately in two directions of the primary winding of the ignition coil, and by detecting this primary current, the period of current interruption is determined thereby to generate a high-frequency ignition voltage across the secondary winding of the ignition coil.
In the conventional ignition system of AC continuous discharge type which uses a couple of power transistors for turning on and off alternately the primary current of the ignition coil at high frequencies, the switching loss (off loss, in particular) of each power transistor generates a considerable amount of heat. This switching loss depends on the interruption frequency of the power transistors and the primary leakage inductance of the ignition coil. If the leakage inductance is reduced, the unit off loss of the power transistors would decrease, but the primary current would start earlier at the time of turning on of the power transistors, so that the interruption frequency of the power transistors would be for increased interruption frequencies of the power transistors. As a result, heat would be generated an increased number of times per unit time with the turning on and off of the power transistors, and the higher level of rise of the primary current would increase the overshoot of the primary current at the time of turning off of the power transistors, with the result that the cut-off current value would be increased for an increased loss of the power transistors. If the leakage inductance is increased, by contrast, in spite of the decreased on-off frequency of the power transistors, the off loss thereof would increase, thereby making it difficult to reduce the total amount of heat generation of the power transistors.
Now, the reason why the on-off frequency of the power transistors is increased by a reduced leakage inductance and the problems caused by this phenomenon will be explained in detail with reference to FIGS. 10A to 12B. FIGS. 10A to FIG. 10D show equivalent circuits of a transformer including the primary and secondary windings of the ignition coil. The equivalent circuits of FIGS. 10A to FIG. 10D are simplified to an increasing degree in that order. A basic equivalent circuit is shown in FIG. 10A. FIG. 10B shows an equivalent circuit using a coupling coefficient K of the transformer for the ignition coil. Further, FIG. 10C shows an equivalent circuit with all the circuit elements transferred to the primary circuit. In these figures, V.sub.b : a source voltage, R.sub.1 : a primary coil resistor, L.sub.1 : a primary inductance, R.sub.2 : a secondary coil resistor, L.sub.2 : a secondary inductance, I.sub.1 : a primary coil current, R.sub.L : a load resistor, N: turn ratio, L.sub.1 '(L.sub.1 (1-k)): a primary leakage inductance, L.sub.2 '(L.sub.2 (1-k)): a secondary leakage inductance. Assuming that R.sub.1 =R.sub.2 /N.sup.2 .div.0 and KL.sub.1 &gt;&gt;(1-k)L.sub.1, in FIG. 10C, on the other hand, the transformer of the ignition coil can be expressed by the simple equivalent circuit of FIG. 10D. As obvious from FIG. 10D, the rising speed of the primary coil current I.sub.1 is determined by the leakage inductances L'.sub.1, L'.sub.2, the load resistor R.sub.L being constant. In controlling the primary coil current of the ignition system of AC continuous discharge type, when the current of a primary winding reaches a predetermined value, the same current is turned off, while the current of the other primary winding is turned on. If a coil of small leakage inductance is used for this type of ignition system, the rising speed of the primary coil current increases, so that the frequency of the primary coil current increases for an increased on-off frequency of the power transistors. This phenomenon is especially conspicuous when the source voltage V.sub.B is high.
The loss P.sub.0 caused at the time of turning off the power transistors is given as P.sub.0 =1/2L'.sub.1, I.sub.P1.sup.2, where L.sub.1 is the primary leakage inductance and I.sub.P1 the cut-off current value of the ignition coil. If the power transistors are turned off a number n of times during a predetermined discharge period, the total loss W.sub.0 during the same period is given as W.sub.0 =nP.sub.0 =1/2nL'.multidot.I.sub.P1.sup.2. With the increase in the rising speed of the primary coil current, that is, frequency, therefore, the on-off frequency n of the power transistors assumes a large value as shown in FIG. 11(b), and at the same time, due to the time delay .tau..sub.1 before the current flowing in the primary winding is detected and interrupted by the power transistors, the cut-off current I.sub.P1 increases to I.sub.P2, with the result that the loss W.sub.0, which is proportional to the square of current, sharply increases. In this way, in the case where the primary coil current rises too early, not only the frequency is increased but also an overshoot of the cut-off current is caused as shown in FIG. 11(b), whereby the loss is increased, thus often breaking the power transistors.
If the frequency of the primary coil current decreases, by contrast, the power transistors are turned off less rapidly as shown in FIG. 12(a) and are operated in an unsaturation region to a corresponding degree thereby to increase the off loss of the power transistors.